The applicant's long-term objective is to understand the role of cellular responses to DNA damage in the pathogenesis of human cancers. The human hereditary disease Xeroderma pigmentosum (XP) provides a prime example of the role of somatic mutations resulting from defective DNA repair, in the pathogenesis of cancer, since afflicted individuals have an extremely high incidence of skin cancers of various types. This particular project is aimed at defining the genetics and molecular pathology of this hereditary human disease. The experimental strategy proposed exploits the highly selectable phenotype of XP cells in culture, based on their well recognized hypersensitivity to killing by DNA-damaging agents. This selectable phenotype is exploited in two ways. Firstly, it is used to identify complementing single human chromosomes, thereby providing the essential first step towards refined genetic mapping of XP loci from various genetic complementation groups. The introduction of single human chromosomes into XP cells is achieved by the technique of microcell-mediated chromosome transfer, using chromosomes tagged with a dominate selectable marker. Secondly, the selectable phenotype of XP cells is used to screen DNA from defined regions of the genome (based on the results of genetic mapping) for complementing sequences, thereby facilitating the molecular cloning of XP genes. The isolation and characterization of wild-type XP genes is expected to provide major insights into the molecular basis of this disease in humans, and to facilitate the establishment of an experimental animal model following molecular disruption of the wild-type mouse gene in cells suitable for implantation into blastocysts.